PREPRINT
9E4D86C3-E077-4B07-94DD-15FFCA7FAACB

# The Spiderweb proto-cluster is being magnetized by its central radio jet

Craig S. Anderson, Christopher L. Carilli, Paolo Tozzi, G. K. Miley, S. Borgani, Tracy Clarke, L. Di Mascolo, Ang Liu, Tony Mroczkowski, Maurilio Pannella, L Pentericci, H. J. A. Rottgering, A. Saro
arXiv:2207.03498

Submitted on 7 July 2022

## Abstract

We present deep broadband radio polarization observations of the Spiderweb radio galaxy (J1140-2629) in a galaxy proto-cluster at $z=2.16$. These yield the most detailed polarimetric maps yet made of a high redshift radio galaxy. The intrinsic polarization angles and Faraday Rotation Measures (RMs) reveal coherent magnetic fields spanning the $\sim 60$ kpc length of the jets, while $\sim 50$% fractional polarizations indicate these fields are well-ordered. Source-frame absolute RM values of $\sim 1,000$ rad/m/m are typical, and values up to $\sim 11,100$ rad/m/m are observed. The Faraday-rotating gas cannot be well-mixed with the synchrotron-emitting gas, or stronger-than-observed depolarization would occur. Nevertheless, an observed spatial coincidence between a localized absolute RM enhancement of $\sim 1,100$ rad/m/m, a bright knot of Ly$\alpha$ emission, and a deviation of the radio jet provide direct evidence for vigorous jet-gas interaction. We detect a large-scale RM gradient totaling $\sim 1,000$s rad/m/m across the width of the jet, suggesting a net clockwise (as viewed from the AGN) toroidal magnetic field component exists at 10s-of-kpc-scales, which we speculate may be associated with the operation of a Poynting-Robertson cosmic battery. We conclude the RMs are mainly generated in a sheath of hot gas around the radio jet, rather than the ambient foreground proto-cluster gas. The estimated magnetic field strength decreases by successive orders-of-magnitude going from the jet hotspots ($\sim 90$ $\mu$G) to the jet sheath ($\sim 10$ $\mu$G) to the ambient intracluster medium ($\sim 1$ $\mu$G). Synthesizing our results, we propose that the Spiderweb radio galaxy is actively magnetizing its surrounding proto-cluster environment, with possible implications for theories of the origin and evolution of cosmic magnetic fields.

## Preprint

Comment: 12 pages, 7 figures, 2 tables. Accepted for publication in ApJ

Subjects: Astrophysics - Astrophysics of Galaxies; Astrophysics - Cosmology and Nongalactic Astrophysics; Astrophysics - High Energy Astrophysical Phenomena